Preheat oven to 400 degree F. Toss the tomatoes, garlic and onion in the olive oil and sprinkle with salt and pepper. Roast on a baking sheet for 40 minutes, or until the tomatoes and onion are soft and charred. Let cool and chop to 1 inch in size. Place the mixture into a colander and press any extra liquid out.
Place the flour, baking powder and butter into a food processor. Pulse until the mixture resembles coarse meal. Transfer to a large bowl and stir in the heavy cream and egg. Mix the dough with a wooden spoon until combined. Add feta cheese, 1 cup of the tomato mixture and basil and gently mix by hand. On a floured surface, form a big dough ball and flatten it until you have a 1 inch thick disk. Cut the disk into 8 equal pieces. Bake for 15 minutes on a baking stone (pizza stone) or a baking sheet lined with parchment paper.
Remove scones and cool on a wire rack.
